As standards of the optical transmission technique to realize the speeding up of the line by hierarchically piling up and multiplexing low-speed lines, SONET/SDH (Synchronous Optical NETwork/ Synchronous Digital Hierarchy) has been standardized.
The frame period in SONET is 125 μs. In addition, the signal format and the like in SONET is hierarchically defined as OC-n (Optical Carrier-level n). Currently, the standardization has been done up to OC-192.
Meanwhile, the frame period in SDH is, in the same manner as the frame period in SONET, 125 μs. In addition, the signal format and the like in SDH is hierarchically defined as STM-n (Synchronous Transmission Module-level n). Currently, the standardization has been done up to M-64.
At OC-3/STM-1 and above, the transmission speed and the hierarchical structure in SONET and SDH are the same.
Here, referring to FIG. 1A-FIG. 1C, the relationship between the frame period and the transmission rate in SONET/SDH is explained. FIG. 1A is a diagram illustrating the relationship between the frame period and the transmission rate at OC-3. FIG. 1B is a diagram illustrating the relationship between the frame period and the transmission rate at OC-12. FIG. 1C is a diagram illustrating the relationship between the frame period and the transmission rate at OC-48. In FIG. 1A-FIG. 1C, the vertical axis represents the frame period, and the horizontal axis represents the transmission rate.
As illustrated in FIG. 1A-FIG. 1C, the size of the frame of OC-n changes depending on n. Specifically, the size of the frame of OC-n is n×90×9 bytes. In addition, as described above, in SONET/SDH, the frame period is constant at 125 μs regardless of the value of n. For this reason, the transmission rate of OC-n is n×51.84 Mbps.
Meanwhile, as a platform for transparent transmission of a client signal, OTN (Optical Transport Network) based on the WDM (Wavelength Division Multiplexing) system has been standardized. Here, the client signal includes, for example, OC-n, STM-n and the like in SONET/SDH, for example. The frame structure of ODUk (Optical Channel Data Unit k) in OTUk (Optical Transport Unit k) defined in OTN is equivalent to OTUk except for FEC (Forward Error Correction) and OH (Overhead).
Here, with reference to FIG. 2, the relationship between the frame period and the transmission rate of ODUk is explained. The vertical axis of FIG. 2 represents the frame period, and the horizontal axis represents the transmission rate. FIG. 2 illustrates the relationship between the frame period and the transmission rate of the ODU0, ODU1, ODU2.
As illustrated in FIG. 2, the size of the frame of ODUk is 3824×4 bytes regardless of the value of k. In contrast, the frame period of ODUk varies depending on the value of k. For example, the frame period of the ODU0 is 98.34 μs, the frame period of the ODU1 is 48.97 μs, and the frame period of the ODU2 is 12.191 μs. Therefore, the transmission rate of ODUk varies depending on the value of k.
The transmission rate of the ODU0 is 1244.16 Mbps. The transmission rate of the ODU1 is 2498.76 Mbps(=1244.16×2×239/238 Mbps). The transmission rate of the ODU2 is 10037.27 Mbps(=1244.16×8×239/237 Mbps). While it is not illustrated in the drawing, the transmission rate of the ODU3 is 40319.21 Mbps(=1244.16×32×239/236 Mbps). The transmission rate of the ODU4 is 104794.45 Mbps(=1244.16×80×239/227 Mbps).
Conventionally, in relation to OTN, a cross-connect apparatus that is capable of providing a clear channel service via networks with different control systems such as between a plurality of carriers has been proposed. The cross-connect apparatus has mapping the client signal in the payload section of an OTN frame, and switching means to perform switching in the ODUk sublayer of the OTN layer for the frame in which the client signal has been mapped.
Patent Document 1: Japanese Laid-open Patent Publication No. 2003-188919